Chronic lymphocytic leukemia (CLL) is a lymphoproliferative disease with a highly variable outcome. The prognosis of patients with CLL may be predicted using a number of biomarkers, including the level of CD38 expression at the leukemic cell surface. This study investigates the hypothesis that CD38 expression by CLL cells reflects interactions with nonmalignant cells within pseudofollicles in secondary lymphoid tissue where tumor cell proliferation is thought to occur. CD38 expression is higher in tissues that contain pseudofollicles compared with those that do not. In addition, we show that CD38 expression in CLL is dynamic, changes in response to contact with activated CD4(+) T cells, and identifies cells that are primed to proliferate. Finally, we demonstrate close contact between activated CD4(+) T cells and proliferating tumor in primary patient tissue. Proliferating tumor cells in lymph nodes express CD38, which is in turn associated with an increased number of CD31(+) vascular endothelial cells. Although the factors resulting in colocalization of tumor, T cells, and endothelium remain unclear, the existence of these cellular clusters may provide an explanation for the association between CD38 expression and adverse outcome in CLL and suggests novel therapeutic targets.